RUSSIA was today condemned after its troops "illegally" crossed the border into Ukraine, despite "hollow denials" from Moscow that no such action had taken place.

Putin faced a rebuke after Russian troops were accused of open participation with rebels in Ukraine[AP]

NATO delivered a stinging rebuke to President Vladimir Putin, as it released photographs claiming to show evidence of Russian artillery units within south-eastern Ukraine.

Moscow has been accused of sending at least 1,000 troops and military equipment into its neighbour's territory, in blatant participation in the ongoing conflict between Ukraine and pro-Russian rebels.

Commenting on the spiralling crisis today, NATO Secretary-General Anders Fogh Rasmussen said: "Despite Moscow's hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine."

He added: "This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation."

Ukraine is now seeking NATO membership in order to earn protection from the alliance.

Earlier today, in a possible move to diffuse tensions, Putin called for pro-Moscow rebels to release Ukrainian soldiers who have become surrounded in the east of the country.

In a statement published on the Kremlin's website, Putin said: "I'm calling on insurgents to open a humanitarian corridor for Ukrainian troops who were surrounded in order to avoid senseless deaths."

He did not comment on the alleged presence of Russian military in Ukraine, instead lauding pro-Russian separatists for "undermining Kiev's military operation which threatened lives of the residents of Donbass and has already led to a colossal death toll among civilians."

Moscow's dismissal of NATO criticism came from Foreign Minister Sergey Lavrov, who brushed off the accusations of Russian military forces entering Ukraine.

He said the Kremlin "has not been presented with any facts" proving that it had happened.

A satellite image released by NATO claims to show Russian self-propelled artillery units in Ukraine [AP]

Russia proved similarly defiant in response to an online jibe from Canada.

Earlier in the week, Canada's delegation to NATO posted a map of Russia and Ukraine, with the latter labelled 'NOT RUSSIA'.

The post read: 'Geography can be tough. Here’s a guide for Russian soldiers who keep getting lost & ‘accidentally’ entering #Ukraine'

However, Russia's delegation responded with their own map, with the recently-annexed Crimea peninsula - of which Russia's sovereignty has not been internationally recognised - labelled 'Russia'.

The post read: 'Helping our Canadian colleagues to catch up with contemporary geography of #Europe'.